Nitride semiconductor substrate and method for manufacturing the same, and nitride semiconductor device using nitride semiconductor substrate

ABSTRACT

A nitride semiconductor substrate including (a) a supporting substrate, (b) a first nitride semiconductor layer having a periodical T-shaped cross-section, having grown from periodically arranged stripe-like, grid-like or island-like portions on the supporting substrate, and (c) a second nitride semiconductor substrate covering said supporting substrate, having grown from the top and side surfaces of said first nitride semiconductor layer, wherein a cavity is formed under the second nitride semiconductor layer. 
     A protective layer having a periodically arranged stripe-like, grid-like or island-like apertures is formed on the supporting substrate. The first nitride semiconductor layer is laterally grown from the exposed portion of the substrate. The growth is stopped before the first nitride semiconductor layer covers the supporting substrate. Thus, the first nitride semiconductor layer has a periodical T-shaped cross-section. Then, the protective layer is removed and the second nitride semiconductor layer is grown from the top and side surface of the first nitride semiconductor layer to cover the substrate.

This application is a division of application Ser. No. 09/880,843, filedJun. 15, 2002, the entire content of which is hereby incorporated byreference in this application.

TECHNICAL FIELD

The present invention relates to a method for growing a nitridesemiconductor (In_(x)Al_(y)Ga_(1−x−y)N, 0≦X, 0≦Y, X+Y≦1), andparticularly to a method for growing a nitride semiconductor which canbe used to make a nitride semiconductor substrate.

BACKGROUND ART

Recently various researches have been conducted on the growth of nitridesemiconductor on a substrate made of a different material such assapphire, spinel or silicon carbide which has a lattice constantdifferent from that of the nitride semiconductor.

For example, a method of growing epitaxial lateral overgrowth GaN (ELOG)is described in JPN. J. Appl. Phys., vol. 37 (1998), pp. L309-L312,wherein nitride semiconductor having lower density of dislocations isobtained by forming a protective film of SiO₂ or other materialpartially on a nitride semiconductor which has been grown on the C planeof sapphire, and growing nitride semiconductor thereon under a reducedpressure of 100 Torr.

In the ELOG growing process, nitride semiconductor having reduceddislocation defects can be formed on the protective film byintentionally growing the nitride semiconductor laterally on theprotective film. When the nitride semiconductor grows, dislocationoccurs and grows only in a window portion of the protective film.

However, in case the protective film of SiO₂ or the like has wide stripewidth, lateral growth of the nitride semiconductor on the protectivefilm does not fully proceed eventually resulting in abnormal growth.

In addition, in case the nitride semiconductor is grown laterally byvapor phase deposition process, while two nitride semiconductor filmswhich grow laterally from the nitride semiconductor exposed on bothsides of the protective film meet and join with each other at the centerof the protective film, dislocations concentrate locally at the joint.This is partly due to the fact that the front surface of the nitridesemiconductor is tilted while growing laterally on the protective filmof SiO₂ or the like. In case a device layer is formed by epitaxialgrowth on a nitride semiconductor substrate such as the above,microscopic pits are likely to be generated in the joint where thedislocations are concentrated. The pits are generated by thedissociation of nitrogen in the process of heating the substrate for thepurpose of growing the device layer. The pits grow larger as theepitaxial growth is continued.

As a result, even when a single continuous nitride semiconductorsubstrate is formed by growing nitride semiconductor layer laterally ona protective film by the vapor phase deposition process, it cannot behandled in the same way as an ordinary single crystal substrate. Sincethe active layer of a semiconductor laser should keep clear of thevicinity of the joint, it is difficult to secure a region large enoughfor forming the device. Moreover, since surface of the single nitridesemiconductor substrate appears to be uniform, it has been difficult torecognize the joint by viewing the top surface of the substrate and tocarry out the formation of device pattern accurately.

Furthermore, in case a single continuous nitride semiconductor substrateis formed by growing nitride semiconductor laterally by using aprotective film on sapphire or the like, such a structure is likely towarp. Because sapphire, the protective film and the nitridesemiconductor layer, which are stacked one on another, have differentcoefficients of thermal expansion.

The different-material substrate may also be removed from the nitridesemiconductor substrate in the last stage. The substrate of differentmaterial maybe removed by polishing or irradiating the interface betweenthe substrate and the nitride semiconductor with excimer laser therebybreaking the chemical bond in the interface. However, it has not beeneasy to remove a different-material substrate such as sapphire as ittakes a long time to remove by polishing or by means of excimer laser.

An object of the present invention is to provide a new structure ofnitride semiconductor substrate manufactured by lateral crystal growthwith a protective film, which is capable of suppressing an adverseeffect caused on the device by joining the nitride semiconductor layerson the protective film. Another object of the present invention is toprevent the nitride semiconductor substrate from warping. Still anotherobject of the present invention is to facilitate removing a substratemade of a different material from the nitride semiconductor substrate.

DISCLOSURE OF THE INVENTION

In order to solve the problems described above, a nitride semiconductorsubstrate according to the first invention comprises (A) a supportingsubstrate, (B) a first nitride semiconductor layer having periodicallyarranged T-shaped cross section formed by laterally growing nitridesemiconductor films starting at portions formed in a periodical stripe,grid or island configuration provided on the surface of the supportingsubstrate and stopping the lateral growth before the films jointogether, and (C) a second nitride semiconductor layer which is grownfrom the top surface or the top and side surface, which side surface hasbeen grown laterally, of the first nitride semiconductor layer as thecore and covers the entire surface of the supporting substrate, whereincavities are formed under the joint of the second nitride semiconductorlayer.

The nitride semiconductor substrate having such a structure as describedabove can be manufactured by (A) forming a protective film havingwindows of stripe, grid or island configuration on the supportingsubstrate, (B) laterally growing the first nitride semiconductor overthe protective film from the exposed portions of the supportingsubstrate and stopping the growth in such a state as the protective filmis not covered, (C) removing the protective film thereby to formcavities below the first nitride semiconductor layer which has beengrown laterally, and (D) growing the second nitride semiconductor layerlaterally from the top surface or the top and side surface, which sidesurface is the portion grown laterally, of the first nitridesemiconductor layer. The supporting substrate may be either a substratemade of a different material such as sapphire or a different-materialsubstrate covered with a nitride semiconductor layer over the entiresurface thereof. In case a substrate made of sapphire or the like isused, it is preferable to form a low temperature-grown buffer layer onthe substrate before growing the first nitride semiconductor. In casethe second nitride semiconductor layer grows from the top surface of thefirst nitride semiconductor layer, the step of removing the protectivefilm may be omitted since both parts of the second nitride semiconductorlayer join with each other above the cavity even when the protectivefilm is not removed.

According to the first aspect of the present invention, the nitridesemiconductor without voids can be grown, even when forming theprotective film widely. Also, strain can be suppressed which wouldotherwise be generated when the second nitride semiconductor is grownfrom the side surface of the first nitride semiconductor, because thesecond nitride semiconductor layer grows over the cavity. Moreover,since the front surface of the growing crystal does not tilt as in thecase of growing on the protective film, concentration of dislocations inthe joint can be relieved.

Also, it is made easier to locate the joint even from above the topsurface of the second nitride semiconductor layer which covers theentire surface of the substrate, because such a cavity exists below thejoint of the second nitride semiconductor layer that has a refractiveindex which is significantly different from that of the nitridesemiconductor. Since the cavity relieves the strain, warping of thesubstrate due to the difference in the thermal expansion coefficientbetween the substrate and the nitride semiconductor layer can bemitigated.

Moreover, because the nitride semiconductor layer is supported by thediscontinuous pillar-like structure on the supporting substrate, bondingstrength between the nitride semiconductor layer and the supportingsubstrate decreases. As a result, not only the conventional methodemploying excimer laser, but also mechanical peeling technique such asvibration or thermal impact maybe used to remove the supportingsubstrate. The supporting substrate can be mechanically peeled off, forexample, by polishing the supporting substrate on the back surfacethereof and making use of the vibration generated during polishing.During polishing, the whole supporting substrate is peeled off by thevibration. When the mechanical peeling technique is employed, thesupporting substrate can be removed in a shorter period of time.Although an interface where the peeling occurs tend to varies, uniformnitride semiconductor substrate can be obtained by polishing thesupporting substrate on the back surface thereof after peeling.

When a different-material substrate covered by the nitride semiconductorlayer is used as the supporting substrate, the nitride semiconductorlayer covering the different-material substrate may be (a) a nitridesemiconductor buffer layer grown at a temperature lower than thetemperature at which the nitride semiconductor layer is to be grownthereafter (hereinafter called low temperature-grown buffer layer); (b)a laminate of a low temperature-grown buffer layer and a gallium nitridelayer; (c) a laminate of a low temperature-grown buffer layer, a galliumnitride layer and an aluminum gallium nitride layer; or (d) a laminateof a low temperature-grown buffer layer, a gallium nitride layer and anindium gallium nitride layer.

Among the constitutions described above, use of nitride semiconductorlayer (c) (=the laminate of low temperature-grown buffer layer, galliumnitride layer and aluminum gallium nitride layer) has an effect ofsuppressing a decomposition of the nitride semiconductor layer on thesupporting substrate surface in the subsequent process thereby toprevent the generation of V-shaped grooves which would otherwise begenerated in the supporting substrate surface. It is also made easier topeel off the supporting substrate by utilizing the stress generated bythe difference in thermal expansion coefficient between the galliumnitride layer and the aluminum gallium nitride layer. When nitridesemiconductor layer (d) (=the laminate of a low temperature-grown bufferlayer, a gallium nitride layer and an indium gallium nitride layer) isused, peeling off the supporting substrate is made easier by utilizingthe fact that the indium gallium nitride layer has mechanical strengthweaker than that of the gallium nitride.

Windows of stripe, grid or island configuration are formed on theprotective film on the supporting substrate. It is preferable to formwindows of grid or island configuration. When the windows of grid orisland configuration are formed, the first nitride semiconductor layergrows in many directions in the plane thus making it easier to peel offthe supporting substrate. It is more preferable to form the windows ofgrid configuration so that the protective film surrounded by the windowhas polygonal or circular shape. When the area of the protective filmsurrounded by the window is formed in polygonal or circular shape, thejoint of the second nitride semiconductor layer becomes a point at thecenter of the protective film thus making it possible to minimize thearea of the joint where dislocations are concentrated.

While the protective film is removed after growing the first nitridesemiconductor layer, the protective film may not be completely removedand it suffices to remove the protective film in such a manner as atleast a cavity is formed under the joint of the second nitridesemiconductor layer. For example, the protective film may be removedonly from under the joint, or just be reduced its thickness.

The protective film may be removed by dry etching or wet etching, ineither way the protective film can be removed without degrading thecrystallinity of the nitride semiconductor. Dry etching is capable ofeasily controlling the depth of the protective film to be removed.

When the protective film is removed so as to expose the surface of thesupporting substrate, the problems caused by the decomposition of theprotective film while growing the nitride semiconductor on theprotective film, namely abnormal growth and degradation of crystallinityof the nitride semiconductor can be mitigated.

The protective film is made of silicon oxide, silicon nitride, titaniumoxide or zirconium oxide, or a multi-layered film of these materials ora film made of a metal having a high melting point not lower than 1200°C. Since such a material for the protective film has the property of notallowing nitride semiconductor to grow easily thereon, the protectivefilm is preferably used for growing the nitride semiconductor laterallythereon.

The nitride semiconductor substrate according to the second inventionhas a nitride semiconductor layer which is grown laterally starting atportions of the substrate formed in a periodical stripe or gridconfiguration provided on the surface of the supporting substrate,wherein the two films of the nitride semiconductor layer grown from therespective starting points do not join with each other but oppose eachother via a clearance.

Thus the nitride semiconductor substrate of the present invention ischaracterized by the configuration of the two films of the nitridesemiconductor layer, grown from the respective starting points, notjoining with each other but disposed to oppose each other via aclearance, on the contrary to the conventional wisdom of the laterallygrown substrate. We found that, even in the case of the nitridesemiconductor substrate whereon the two films of the nitridesemiconductor layer grown laterally are disposed to oppose each othervia a clearance, crystal to make such a device as laser or LED can begrown flatly by the vapor phase epitaxial process. We also found that,since the epitaxial growth is started in such a state as there is nojoint where dislocations would be concentrated, there occurs nogeneration of pits due to the dissociation of nitrogen when thesubstrate is heated which has been a problem in the prior art. Further,it is made possible to grow a flat device layer having bettercrystallinity than those obtained by the prior art.

The nitride semiconductor substrate having such a structure as describedabove can be manufactured by, for example, forming a protective filmhaving stripe, grid or island-like configuration on the supportingsubstrate, laterally growing the nitride semiconductor over theprotective film from the exposed portions of the supporting substrateand stopping the growth in such a state as the protective film is notcompletely covered. The supporting substrate maybe either a substratemade of a different material such as sapphire or a different-materialsubstrate covered with a nitride semiconductor layer over the entiresurface thereof.

It is preferable to form cavities by removing the protective film underthe laterally-grown nitride semiconductor. Forming the cavities makes iteasier to locate the clearance in the subsequent device forming process.Also the strain generated by difference of the coefficients of thermalexpansion between the different-material substrate and the nitridesemiconductor can be mitigated, thereby to suppress the warping of thenitride semiconductor substrate. Preferable structure and composition ofthe supporting substrate, material and shape of the protective film andmethod of removing the protective film are similar to those of the firstinvestment.

The nitride semiconductor substrate of the present invention maycomprises (A) the nitride semiconductor layer having low density ofdislocations obtained according to the first invention or the secondinvention, (B) a thick nitride semiconductor layer for dispersingdislocations of the nitride semiconductor layer which has grown byhalide vapor phase epitaxy process (hereinafter called the HVPEprocess), and (C) a nitride semiconductor layer formed by the similarway as the first invention or the second invention. In the nitridesemiconductor layer obtained according to the first invention or thesecond invention, dislocations remain above the windows of theprotective film. By dispersing the dislocations by means of the thicknitride semiconductor layer formed by the HVPE growing process, thenitride semiconductor layer can be made to have a relatively lowdislocation density over the whole area. By growing the layers accordingto the first invention or the second invention on the base of the HVPEgrown nitride semiconductor layer, a nitride semiconductor substratehaving even lower density of dislocations can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A through FIG. 1D are schematic sectional views showing a methodfor manufacturing a nitride semiconductor substrate of Embodiment 1.

FIG. 2A through FIG. 2C are schematic drawings showing the pattern of aprotective film.

FIG. 3A through FIG. 3C are schematic drawings showing the pattern of aprotective film.

FIG. 4 is a plan view on the principal plane side of a substrate showingthat the stripe protective film is formed in such a state as thedirection of stripe is offset a little from the orientation flatsurface.

FIG. 5 is a sectional view schematically showing another example of thenitride semiconductor of Embodiment 1.

FIG. 6 is a sectional view schematically showing still another exampleof the nitride semiconductor substrate of Embodiment 1.

FIG. 7 is a sectional view schematically showing yet another example ofthe nitride semiconductor substrate of Embodiment 1.

FIG. 8 is a schematic sectional view showing a process for manufacturinga nitride semiconductor substrate of Embodiment 2.

FIGS. 9A and 9B are sectional views schematically showing amanufacturing process to form a device on the nitride semiconductorsubstrate shown in FIG. 8.

FIG. 10 is a sectional view schematically showing a nitridesemiconductor device using a nitride semiconductor substrate of anotherexample of Embodiment 2.

FIG. 11 is a sectional view schematically showing a nitridesemiconductor device using a nitride semiconductor substrate of stillanother example of Embodiment 2.

FIGS. 12A and 12B are sectional views schematically showing a nitridesemiconductor substrate of Embodiment 3.

FIG. 13 is a schematic sectional view of the nitride semiconductorsubstrate in FIG. 6 showing more detail of the joint portion.

FIG. 14A and FIG. 14B are CL images of the nitride semiconductorsubstrate in Example 6 (FIG. 14A) and Comparable example 1 (FIG. 14B).

BEST MODE FOR CARRYING OUT THE INVENTION

Now the present invention will be described in detail below withreference to the accompanying drawings.

Embodiment 1

The first embodiment will be described below in relation to the nitridesemiconductor substrate according to the first invention. FIG. 1Athrough FIG. 1D are schematic drawings showing stepwise an example ofmethod for manufacturing the nitride semiconductor substrate accordingto the first invention.

FIG. 1A is a schematic sectional view showing the process of growing thenitride semiconductor on a different-material substrate 1 and formingstripes of the protective film.

The substrate 1 of different material may be made of an insulatingsubstance such as sapphire or spinel (MgAl₂O₄) having principal plane inthe C plane, R plane or A plane, or SiC (6H, 4H, 3C), ZnS, ZnO, GaAs, Sior an oxide that has lattice constant similar to the nitridesemiconductor.

Alternatively, a buffer layer (not shown) may be formed on the substrate1 before growing the nitride semiconductor 2 on the substrate 1. Thebuffer layer may be AlN, GaN, AlGaN, InGaN or the like. The buffer layeris grown to a thickness in a range from 0.5 μm to 10 Å at a temperaturein a range from 300 to 900° C. This is for the purpose of mitigating themismatch of lattice constant between the substrate 1 and the nitridesemiconductor 2, and is preferable in order to reduce crystal defects.

The nitride semiconductor 2 formed on the substrate 1 may be made ofundoped GaN or GaN doped with an n-type impurity such as Si, Ge, Sn orS. The nitride semiconductor 2 is formed on the substrate 1 at atemperature in a range from 900 to 1000° C. to a thickness preferably1.5 μm or greater which makes it possible to form a specular surfacewith less pits on the crystal surface and is therefore desirable. Thenitride semiconductor 2 may also be formed by stacking a GaN film and anAl_(x)Ga_(1−x)N film (0<x<1, preferably 0<x≦0.5) or stacking a GaN filmand an In_(y)Ga_(1−y)N film (0<y≦1). Using these constitutions makes iteasier to remove the substrate 1 by making use of the stress generatedby the difference in the thermal expansion coefficient between the GaNfilm and the Al_(x)Ga_(1−x)N film and the low strength of theIn_(xy)Ga_(1−y)N film. In this case, the Al_(x)Ga_(1−x)N film and theIn_(xy)Ga_(1−y)N film may be doped with an n-type impurity or undoped.

The protective film 3 to be formed on a part of the surface of thenitride semiconductor 2 is made of a material which does not allow thenitride semiconductor to grow easily thereon. Preferably the protectivefilm is made of an oxide or a nitride such as silicon oxide (SiO_(x)),silicon nitride (Si_(x)N_(y)), titanium oxide (TiO_(x)) or zirconiumoxide (ZrO_(x)) , or a multi-layered film of these materials.

In addition to the materials described above, a metal having a meltingpoint not lower than 1200° C. such as tungsten and molybdenum may beused.

The protective film 3 is formed by CVD, sputtering or vapor depositionand then, with a resist film coated thereon, etched to form a stripe orgrid configuration by photolithography process. By etching theprotective film into a stripe or grid configuration, windows in theshape of stripes or islands are formed in the protective film.Alternatively, the protective film 3 may also be left to remain in theconfiguration of islands to form the windows of grid configuration inthe protective film 3. Width of the stripe or grid of the protectivefilm is not limited, while it is preferably in a range from 5 to 20 μmin the case of stripe. The window or opening in the protective film 3preferably smaller than the stripe width. When the protective film isformed in the configuration of islands to form windows of gridconfiguration, the islands are made to a width of 10 μm or less,preferably 5 μm or less, and the grid-shaped windows are made to a widthin a range from 10 to 30 μm, preferably from 10 to 20 μm.

There is no limitation to the thickness of the protective film since itis not necessary to form the first nitride semiconductor so as to coverthe protective film completely, and the thickness may be in a range from0.05 to 10 μm.

FIG. 2A through FIG. 2C and FIG. 3A through FIG. 3C are ton views of thesubstrate after etching. FIG. 2A shows a case where the protective film3 to be formed on the nitride semiconductor 2 is etched in stripeconfiguration. FIG. 2B and FIG. 2C show cases where the protective film3 is etched in grid configuration to form island-like windows. Theisland-like windows may be formed in polygon (triangle, rectangle,hexagon, etc.) as shown in FIG. 2B and FIG. 2C, or circle.

FIG. 3B and FIG. 3C show cases where the protective film 3 is left toremain in the configuration of islands to form the windows of gridconfiguration. The protective film 3 may be formed in polygon (triangle,rectangle, hexagon, etc.) as shown in FIG. 3A and FIG. 3C, or circle asshown in FIG. 3B. The islands of the protective film 3 are preferablydisposed in a dense arrangement at constant intervals as far aspossible. In FIG. 3A, for example, hexagonal islands of the protectivefilm 3 are arranged in honeycomb configuration (one side each ofadjacent hexagons oppose each other, with each hexagon surrounded by sixhexagons), while in FIG. 3C one side each of adjacent triangle opposeeach other while six triangles form one hexagon and the hexagons aredisposed in honeycomb arrangement. With these arrangements, distancebetween the islands of the protective film 3 (width of window) can bemade uniform and the islands of the protective film 3 can be arrangedwith a high density. The protective film 3 is not limited to theconfiguration shown in FIG. 2 and FIG. 3, and may have any configurationas long as the nitride semiconductor 2 is exposed periodically.

Forming the windows in the protective film 3 in the configuration ofislands as shown in FIG. 2A and FIG. 2C or grid as shown in FIG. 3Athrough FIG. 3C has such an advantage as it is made easier to peel offthe supporting substrate under the first nitride semiconductor layer 4since the subsequent growth of the first nitride semiconductor layer 4occurs in many direction (indicated by arrows in the drawing).

Forming the windows of grid configuration by leaving the protective film3 in the configuration of islands as shown in FIG. 3 has such anadvantage that the joint of the second nitride semiconductor layer 5 tobe grown later becomes a point at the center of the protective film 3thus making it possible to minimize the area of the joint wheredislocations are concentrated.

In case the protective film is formed in stripes, good crystal of flatgrowth surface can be obtained by arranging the stripes as shown in FIG.4 with the orientation flat surface being set in the A plane of sapphirewith the direction of growth being set at an angle θ=0.1 to 1° to theleft or right from the normal direction of the orientation flat surface.

Then as shown in FIG. 1B, the first nitride semiconductor 4 is grownthrough the windows in the protective film using the nitridesemiconductor 2 as the core, and the lateral growth of the first nitridesemiconductor 4 on the protective film 3 is stopped before theprotective film 3 is completely covered. The first nitride semiconductor4 which has grown in this way has a cross sectional of periodical Tconfiguration as shown in FIG. 1B.

While there is no limitation to the first nitride semiconductor 4 to begrown on the nitride semiconductor 2 formed on the protective film 3,nitride semiconductor of GaN is preferable.

The first nitride semiconductor 4 may be doped with p-type impurity orn-type impurity, or undoped.

Preferable thickness of the first nitride semiconductor 4 depends on thethickness and size of the protective film 3. Since it is necessary tohave a portion of good crystallinity laterally grown on the surface ofthe protective film, the first nitride semiconductor 4 is preferablygrown to have a thickness at least 1.5 times larger that of theprotective film and in a range from 1.5 to 2 μm.

Then as shown in FIG. 1C, in the state that lateral growth of the firstnitride semiconductor 4 on the protective film 3 is stopped midway, theprotective film is removed.

The protective film can be removed by etching. While there is nolimitation to the etching process, dry etching or wet etching may beemployed. Isotropic dry etching will make it easier to control theetching process.

By removing the protective film, a cavity is formed under a portion ofthe first nitride semiconductor 4 which has grown laterally. As aresult, in the nitride semiconductor which is grown on the first nitridesemiconductor 4, generation of stress in the interface with theprotective film during growth from the side surface of the first nitridesemiconductor.

Then as shown in FIG. 1D, on the first nitride semiconductor 4 fromwhich the protective film 3 has been removed, a second nitridesemiconductor 5 is grown from the top surface and the side surface ofthe first nitride semiconductor 4.

The second nitride semiconductor 5 may be made of undoped GaN, GaN dopedwith an n-type impurity such as Si, Ge, Sn or S or GaN doped with anp-type impurity such as Mg. The second nitride semiconductor 5 is grownat a temperature in a range from 900 to 1100° C. Among the materialsdescribed above, Mg-doped makes the second nitride semiconductor 5easier to fill the clearance in the first nitride semiconductor 4 and ispreferable. On the other hand, undope provides a stable electricproperty. Since the second nitride semiconductor 5 grows above thecavity, Al_(x)Ga_(1−x)N (0<x<1) may also be used which cannot be used inthe prior art because of low selectivity for the growth on a protectivefilm.

Thickness of the second nitride semiconductor 5 is preferably in a rangefrom 5 to 20 μm in the case of GaN, and in a range from 2 to 15 μm inthe case of Al_(x)Ga_(1−x)N.

The second nitride semiconductor layer 5 may have multi-layeredstructure, which is preferably a super-lattice structure. The thicknessof each layer is preferably 10 Å˜2 μm. When the second nitridesemiconductor layer 5 is multi-layered, it may also play a role as afunctional layer of a device, for example, contact layer or evencladding layer. This reduces the whole thickness of the device and,therefore, suppress warping of the device. Another advantage of themulti-layered structure is preventing dislocations from proceeding inthe upright direction. GaN/Al_(x)Ga_(1−x)N (0<x<1) multi layer ispreferable for the second nitride semiconductor 5, becauseAl_(x)Ga_(1−x)N is grown under such a condition as accelerating thelateral growth and effectively prevents the upward proceeding ofdislocations. For example, GaN and Al_(x)Ga_(1−x)N pair each having 200Å thickness is repeated 50 times as super lattice layer.

Since the second nitride semiconductor 5 is grown from the top and sidesurface of the first nitride semiconductor having good crystallinitywhich has been obtained by lateral growth, crystal defects areeliminated in the second nitride semiconductor, with crystal defectsremaining only above the windows of the protective film 3. While FIG. 1Dshows a case of growing the second nitride semiconductor 5 laterallyfrom the top surface and the side surface of the first nitridesemiconductor 4 as the core, the second nitride semiconductor 5 may alsobe grown only from the top surface of the first nitride semiconductor 4.When the second nitride semiconductor 5 is grown from the top surface ofthe first nitride semiconductor 4, the step of removing the protectivefilm may be omitted since both parts of the second nitride semiconductorlayer join with each other above the cavity even when the protectivefilm is not removed.

By removing the protective film 3 completely so as to expose the nitridesemiconductor 2 as shown in FIG. 5, such a trouble can be prevented asthe protective film made of SiO₂ or the like decomposes at a temperatureabove 1000° C. and diffuses into the nitride semiconductor located onthe protective film when a device is formed on the substrate. Thus sucha problem is solved as the intrusion of decomposed SiO₂ into the nitridesemiconductor causes degradation of crystallinity or abnormal growth.

Also even in the case of growing the second nitride semiconductor 5laterally from the top surface and the side surface of the first nitridesemiconductor with the protective film having been completely removed,cavities remain in the second nitride semiconductor and suppress thepropagation of the crystal defects from the nitride semiconductor 2which includes many crystal defects.

As shown in FIG. 6, the protective film 3 may be removed until thenitride semiconductor 2 is exposed with a part of the protective film 3,like column, being left under a portion of the laterally-grown firstnitride semiconductor 4. In this case, too, such a trouble can beprevented as the protective film made of SiO₂ or the like decomposes ata temperature above 1000° C. and diffuses into the nitride semiconductorlocated on the protective film when a reactive device is formed on thesubstrate, which leads to degradation of crystallinity or abnormalgrowth. In the aspect of the invention shown in FIG. 5 and FIG. 6,surface of the nitride semiconductor 2 exposed through the protectivefilm 3 decomposes during the process, and V-shaped grooves tend to begenerated in the nitride semiconductor 2. Formation of the V-shapedgrooves due to decomposition of the nitride semiconductor 2 may causecontamination of the first and second nitride semiconductors 4, 5.Formation of the V-shaped grooves, however, may contributes to making iteasier to peel off the supporting substrate, and also to suppressdislocations In the joint of second nitride semiconductors 5. In orderto form the V-shaped grooves intentionally, surface of the nitridesemiconductor layer 2 is preferably gallium nitride or indium galliumnitride. In order to suppress the formation of the V-shaped grooves,surface of the nitride semiconductor layer 2 is preferably aluminumgallium nitride.

The nitride semiconductor substrate may also be made by growing theprotective film 3 without growing the nitride semiconductor 2 on thesubstrate 1 as shown in FIG. 7.

The nitride semiconductor substrate according to this embodiment hassuch features that (1) the concentration of dislocations in the joint ofthe nitride semiconductor is mitigated, (2) it is easy to locate thejoint and (3) warping is suppressed. Therefore, it is made easy tomanufacture nitride semiconductor devices such as semiconductor laser.When a semiconductor laser is manufactured, stripes provided for thepurpose of controlling the transverse mode of the semiconductor laserare preferably formed in an area between the starting point of growingthe first nitride semiconductor 4 and the joint of the second nitridesemiconductor 5. The number of dislocation in this area is not more than10⁷ cm⁻². This is because the region which has becomes the startingpoint of growing the first nitride semiconductor 4, namely the windowregion of the protective film 3 has a high density of dislocations andthe joint of the second nitride semiconductor 5 has a higher density ofdislocations than the other portions though significantly made lowercompared to the prior art. Ridges in the case of a ridge wave guidesemiconductor laser, or buried stripes in the case of a buriedhetero-junction semiconductor laser, are formed in an area between thestarting point of growing the first nitride semiconductor 4 and thejoint of the second nitride semiconductor 5. Since dislocation densityof the joint region is made lower compared to the prior art, it ispossible to form the stripe configuration of the laser much nearer tothe joint, thus extending a life of the laser device.

Embodiment 2

The second embodiment will be described below in relation to the nitridesemiconductor substrate according to the second invention. FIG. 8Athrough FIG. 8C show an example of a method for manufacturing thenitride semiconductor substrate according to the second invention. Theprocess shown in FIG. 8A through FIG. 8C is similar to that of the firstembodiment shown in FIG. 1A through FIG. 1C, and the manufacturingconditions described in conjunction with FIG. 1A through FIG. 1C can beapplied to this process. FIG. 8A is a schematic sectional view showingthe process of growing the nitride semiconductor on the substrate 1 madeof different material and forming stripes of the protective film. Abuffer layer (not shown) may be formed on the substrate 1 before growingthe nitride semiconductor 2 on the substrate 1.

Then as shown in FIG. 8B, the first nitride semiconductor 4 is grownthrough the windows in the protective film using the nitridesemiconductor 2 as the core, and the lateral growth of the first nitridesemiconductor 4 on the protective film 3 is stopped before two areas ofthe first nitride semiconductor 4 growing from adjacent windows joinwith each other and the protective film 3 is completely covered.

Although the nitride semiconductor substrate in the state of FIG. 8B maybe used as the substrate, it is more preferable to remove the protectivefilm 3 as shown in FIG. 8C. By removing the protective film 3 so as toexpose the nitride semiconductor 2, such a trouble can be prevented asthe protective film made of SiO₂ or the like decomposes at a temperatureabove 1000° C. and diffuses into the nitride semiconductor located onthe protective film when a reactive device is formed on the substrate.Thus such a problem is solved as the intrusion of decomposed SiO₂ intothe nitride semiconductor causes degradation of crystallinity orabnormal growth. Also since the cavities are formed under the firstnitride semiconductor 4 after forming the device as the protective film3 is removed, the device pattern can be formed while recognizing theclearance 4 a. Moreover, the nitride semiconductor substrate can berestricted from warping by relieving the strain between the substrate 1and the nitride semiconductor layer 4.

The nitride semiconductor substrate formed as described above has such aconfiguration as separate portions of the nitride semiconductor layer 4which have grown laterally do not join each other and the nitridesemiconductor layer 4 has the cross section of periodical T shape. Thatis, while there is the clearance 4 a between separate portions of thenitride semiconductor layer 4 on the top layer of the substrate, thenitride semiconductor layer can be epitaxially grown in a flatconfiguration even on the nitride semiconductor layer which is not acontinuous sheet.

FIGS. 9A and 9B are schematic drawings showing the process ofmanufacturing the nitride semiconductor device by epitaxially growing adevice forming layer on the nitride semiconductor substrate obtained bythe method shown in FIG. 8. First, as shown in FIG. 9A, the nitridesemiconductor substrate obtained by the method shown in FIG. 8 is put ina vapor phase epitaxial growing apparatus, and is heated to about 900 to1200° C. which is suited for growing the nitride semiconductor. In thisheating process, pits are not generated in the nitride semiconductorsubstrate of this embodiment since the separate portions of the nitridesemiconductor layer 4 which have grown laterally do not join each other.In the prior art process, pits are generated in the surface of thenitride semiconductor substrate before reaching the growing temperatureof the nitride semiconductor because of dissociation of nitrogen in thejoint of lateral growth where dislocations are concentrated.

Then as shown in FIG. 9B, an n-type contact layer 6 is formed directlyon the nitride semiconductor substrate, and a nitride semiconductorlayer 7 including an n-type cladding layer, an active layer and a p-typecladding layer is formed by continuous epitaxial growth. The n-typecontact layer 6 and the nitride semiconductor layer 7 formed thereonconstitute the nitride semiconductor device such as laser or LED. Byforming the n-type contact layer 6 to a large thickness, the clearancein the nitride semiconductor layer 4 can be filled in thereby to form aflat surface. The n-type contact layer 6 can be made of, for example,Al_(x)Ga_(1−x)N (0≦x<0.5) of which thickness is preferably in a rangefrom 5 to 10 μm. The entire device forming layer including the n-typecontact layer 6 is preferably grown continuously while keeping thenitride semiconductor growing temperature in a range from 900 to 1200°C. Instead of forming the n-type contact layer 6 directly on the nitridesemiconductor substrate, the n-type contact layer 6 may also be formedafter growing a buffer layer made of nitride semiconductor such as GaNon the nitride semiconductor substrate at temperature in a range from900 to 1200° C. Also instead of directly forming the n-type contactlayer 6, the n-type contact layer 6 may also be formed after growing anitride semiconductor layer (preferably GaN layer) doped with Mg. Sincethe nitride semiconductor layer doped with Mg is easier to growlaterally, the clearance 4 a in the nitride semiconductor substrate canbe efficiently filled in.

When a semiconductor laser is constituted from the n-type contact layer6 and the nitride semiconductor layer 7, stripes provided for thepurpose of controlling the transverse mode of the semiconductor laserare preferably formed in an area between the starting point of growingthe first nitride semiconductor 4 and the joint of the second nitridesemiconductor 5. The number of dislocation in this area is not more than10⁷ cm⁻². This is because the region which has becomes the startingpoint of growing the first nitride semiconductor 4, namely the window ofthe protective film 3 has a high density of dislocations and the centerof the clearance 4 a between adjacent portions of the nitridesemiconductor 4 has a higher density of dislocations than the otherportions. Ridges in the case of a ridge waveguide semiconductor laser,or buried stripes in the case of a buried hetero-junction semiconductorlaser, are formed between the starting point of growing the firstnitride semiconductor 4 and the center of the clearance 4 a of thenitride semiconductor layer 4.

While it is preferable to completely remove the protective film 3 madeof SiO₂ or the like so as to expose the nitride semiconductor 2 as shownin FIG. 8C, a part of the protective film 3 may be left like column toremain below the portion of the first nitride semiconductor 4 as shownin FIG. 10. In this case, too, such a trouble can be prevented as theprotective film made of SiO₂ or the like decomposes at a temperatureabove 1000° C. and diffuses into the nitride semiconductors 6, 7 locatedon the protective film 3 thus leading to degradation of crystallinity orabnormal growth when a reactive device is formed on the substrate.

The nitride semiconductor substrate may also be manufactured bylaterally growing the first nitride semiconductor layer 4 directlywithout growing the nitride semiconductor 2 on the substrate 1 andstopping the lateral growth before the entire surface of the substrateis covered. Preferable structure and composition of the supportingsubstrate, material and shape of the protective film and method ofremoving the protective film are similar to those of the firstinvestment.

Embodiment 3

FIG. 12A and FIG. 12B are schematic sectional view sowing a nitridesemiconductor substrate according to the third embodiment of the presentinvention. In this embodiment, a thick nitride semiconductor layer 8 isgrown by the HVPE process for dispersing dislocations on a nitridesemiconductor layer which is obtained by the process of the first orsecond embodiment (hereinafter first lateral growth), then a nitridesemiconductor layer is grown by a method similar to those of the firstor second embodiment (hereinafter second lateral growth), therebyconstituting the nitride semiconductor substrate.

The first and the second lateral growth may be carried out by the methodof either the first embodiment or the second embodiment. They may alsobe combined in any of four possible combinations. Two of these will bedescribed below with reference to FIG. 12A and FIG. 12B.

FIG. 12A shows an example of the first lateral growth being carried outby a method similar to that of the first embodiment and the secondlateral growth being carried out by a method similar to that of thesecond embodiment. The process is similar to that of the firstembodiment up to the point of forming the nitride semiconductor layer 2,the first nitride semiconductor layer 4 and the second nitridesemiconductor layer 5 on the substrate 1 made of sapphire or the like.Then the thick HVPE layer 8 is formed on the second nitridesemiconductor layer 5. While dislocations are concentrated in a region 5b of the second nitride semiconductor layer 5 located above the windowof the protective film 3, the dislocations are uniformly dispersed overthe entire HVPE layer 8 due to the HVPE layer 8 being formed with alarge thickness. Then the nitride semiconductor layer 4′ having T-shapedcross section is formed on the HVPE layer 8 by a method similar to thatof the second embodiment, and further the device forming layers 6 and 7are formed.

FIG. 12B shows an example of the first lateral growth being carried outby a method similar to that of the second embodiment and the secondlateral growth being carried out by a method similar to that of thefirst embodiment. The process is similar to that of the secondembodiment up to the point of forming the nitride semiconductor layer 2,the first nitride semiconductor layer 4 having T-shaped cross section onthe substrate 1 made of sapphire or the like. Then the thick HVPE layer8 is formed on the nitride semiconductor layer 4 having T-shaped crosssection. While dislocations are concentrated in a region 4 b of thenitride semiconductor layer 4 located above the window of the protectivefilm 3, the dislocations are uniformly dispersed over the entire HVPElayer 8 due to the HVPE layer 8 being formed with a large thickness.Then the first nitride semiconductor layer 4′ and the second nitridesemiconductor layer 5′ are formed on the HVPE layer 8 by a methodsimilar to that of the first embodiment, and further the device forminglayers 6 and 7 are formed.

According to this embodiment, dislocations which remain in the nitridesemiconductor layer obtained by the first lateral growth are disperseduniformly by the thick nitride semiconductor layer 8 formed by the HVPRgrowing process and the second lateral growth is carried out on the baseof the nitride semiconductor layer 8, thereby to obtain the nitridesemiconductor substrate having even lower density of dislocations. ThickHVPE layer 8 is preferable to disperse the dislocation uniformly. Thethickness of the HVPE layer is at least 10 μm, preferably not less than50 μm, more preferably not less than 200 μm, still more preferably notless than 400 μm.

In the method for manufacturing the nitride semiconductor substrate ofthe present invention, there is no limitation to the method of growingthe nitride semiconductor 2, the first nitride semiconductor 4 and thesecond nitride semiconductor. For example, such methods may be employedas MOVPE (metal-organic vapor phase deposition), HVPE (halide vaporphase deposition), MBE (molecular beam epitaxy) and MOCVD (metal-organicchemical vapor phase deposition).

While dry etching or wet etching may be employed for forming the windowsin the protective film and removing the protective film, anisotropicetching is preferably employed for forming the windows and isotropicetching is preferably employed for removing the protective film.

Now Examples of the present invention will be described below, but theinvention is not limited to these embodiments.

EXAMPLE 1

A sapphire substrate 1 having principal plane in C plane and orientationflat surface in A plane is used. A buffer layer of GaN is formed to athickness of 200 Å on the sapphire substrate 1 at a temperature of 510°C. by MOCVD process using hydrogen as the carrier gas and ammonia andTMG (trimethyl-gallium) as the stock material gas.

After growing the buffer layer, supply of only the TMG is stopped andthe temperature is raised to 1050° C. When the temperature has reached1050° C. nitride semiconductor 2 made of undoped GaN is grown to athickness of 2.5 μm by using TMG, ammonia and silane gas as the stockmaterial gas.

A protective film 3 made of SiO₂ is formed to a thickness of 0.5 μm bythe CVD process on the nitride semiconductor 2 and, after forming aphoto-mask of stripe configuration, the protective film 3 made of SiO₂having stripe width of 14 μm and window size of 6 μm is formed byetching. Direction of the stripes of the protective film 3 is setperpendicular to the sapphire A plane. The clearance of the twoadjuscent nitride semiconductor 4 is about 2 μm.

Then the first nitride semiconductor 4 made of GaN is grown to athickness of 2 μm at a temperature of 1050° C. under a reduced pressureby MOCVD process using TMG, ammonia, silane gas and Cp₂Mg(cyclopentadienyl magnesium) as the stock material gas. In this process,the first nitride semiconductor 4 is grown from the window of the SiO₂protective film and laterally grown on the protective film. The growthis stopped before the first nitride semiconductor 4 completely coversthe SiO₂ protective film.

Then the SiO₂ protective film is etched to a depth of 0.3 μm byisotropic dry etching at a temperature of 120° C. using oxygen and CF4as the etching gas.

The second nitride semiconductor 5 made of GaN is grown, from the sidesurface and the top surface of the first nitride semiconductor which hasbeen laterally grown, to a thickness of 15 μm at a temperature of 1050°C. under atmospheric pressure by the MOCVD process using TMG, ammonia,silane gas and Cp₂Mg (cyclopentadienyl magnesium) as the stock materialgas. The reduced pressure may also be employed to grow the secondnitride semiconductor 5.

CL (cathode luminescence) observation of the surface of the secondnitride semiconductor 5 made as described above shows crystal defectsover the windows of the protective film. However, few crystal defectsare observed on the surface of the second nitride semiconductor 5 grownover the protective film, showing good crystallinity. The number of thedefects is about 6×10⁶ cm⁻².

EXAMPLE 2

A sapphire substrate 1 having principal plane in C plane and orientationflat surface in A plane is used. A buffer layer of GaN is formed to athickness of 200 Å on the sapphire substrate 1 at a temperature of 510°C. by the MOCVD process using hydrogen as the carrier gas and ammoniaand TMG (trimethyl-gallium) as the stock material gas.

After growing the buffer layer, a protective film 3 made of SiO₂ isformed on the buffer layer to a thickness of 0.5 μm by the CVD processand, after forming a photo-mask of stripe configuration, the protectivefilm made of SiO₂ having stripe width of 14 μm and window size of 6 μmis formed by etching. Direction of the stripes of the protective film 3is set perpendicular to the sapphire A plane.

Then the first nitride semiconductor 4 made of GaN is grown to athickness of 15 μm at a temperature of 1050° C. under a reduced pressureby the MOCVD process using TMG, ammonia, silane gas and Cp₂Mg(cyclopentadienyl magnesium) as the stock material gas. In this processthe first nitride semiconductor 4 is grown from the window of the SiO₂protective film and laterally grown on the protective film. The growthis stopped before the first nitride semiconductor completely covers theSiO₂ protective film. The clearance of the two adjacent nitridesemiconductor 4 is about 2 μm.

Then the SiO₂ protective film 3 is etched to a depth of 0.3 μm byisotropic etching at a temperature of 120° C. using oxygen and CF₄ asthe etching gas.

The second nitride semiconductor 5 made of GaN is grown, from the sidesurface and the top surface of the first nitride semiconductor, to athickness of 15 μm at a temperature of 1050° C. under atmosphericpressure by the MOCVD process using TMG, ammonia, silane gas and Cp₂Mg(cyclopentadienyl magnesium) as the stock material gas.

CL (cathode luminescence) observation of the surface of the secondnitride semiconductor 5 made as described above provided similar resultsto Example 1.

EXAMPLE 3

The second nitride semiconductor is grown similarly to Example 1, exceptfor etching the protective film till the first nitride semiconductor isexposed.

Satisfactory result substantially similar to Example 1 is obtained.

EXAMPLE 4

The second nitride semiconductor is grown similarly to Example 2, exceptfor etching the protective film till the sapphire of the substrate isexposed.

Satisfactory result substantially similar to Example 2 is obtained.

EXAMPLE 5

The second nitride semiconductor is grown similarly to Example 2, exceptfor growing the buffer layer after forming the protective film directlyon the substrate. That is, with a sapphire substrate 1 having principalplane in C plane and orientation flat surface in A plane being used, aprotective film made of SiO₂ is formed to a thickness of 0.5 μm by theCVD process on the sapphire substrate and, after forming a photo-mask ofstripe configuration, the protective film made of SiO₂ having stripewidth of 14 μm and window size of 6 μm is formed by etching. Directionof the stripes of the protective film 3 is set perpendicular to thesapphire A plane.

Then a buffer layer of GaN is formed to a thickness of 200 Å on thesapphire substrate 1 at a temperature of 510° C. by the MOCVD processusing hydrogen as the carrier gas and ammonia and TMG(trimethyl-gallium) as the stock material gas. Then the first nitridesemiconductor 4 made of GaN is grown to a thickness of 15 μm at atemperature of 1050° C. under a reduced pressure by the MOCVD processusing TMG, ammonia, silane gas and Cp₂Mg (cyclopentadienyl magnesium) asthe stock material gas.

In this case, while the buffer layer grows to a certain extent on theSiO₂ protective film as well on the sapphire, although the buffer layeron the SiO₂ film has poor film quality. As a result, the first nitridesemiconductor 4 starts growing only from the portion of the buffer layerwhich has grown on the sapphire (window of the SiO₂) and grows laterallysimilarly to Example 2. Thus satisfactory result similar to Example 2 isobtained.

EXAMPLE 6

The nitride semiconductor substrate is grown similarly to Example 1,except for etching the protective film till the sapphire of thesubstrate is exposed while remaining a part of the protective film likecolumn under the canopy part of the laterally-grown nitridesemiconductor 4. The width of the protective film remained at both sideof the leg part of the nitride semiconductor 4 is about 3.5 μm. Theprotective film is etched by anisotropy etching at 200° C. with CHF₃.

CL (cathode luminescence) observation of the surface of the secondnitride semiconductor 5 is shown in FIG. 14A. While defects are observedover the windows of the protective film, few crystal defects areobserved on the surface of the second nitride semiconductor 5 grown overthe protective film except the joint portion. The number of the defectsis about 6×10⁶ cm⁻². At the joint portion, a few defects are observed,but the amount of the defects is much smaller than the prior art.

EXAMPLE 7

The nitride semiconductor substrate is grown similarly to Example 6,except for forming the protective film in hexagon arranged in honeycombconfiguration as shown in FIG. 3A. The protective film is arranged sothat the side of the hexagon is parallel to the orientation flat surface(A plane). The diameter A of the hexagon is about 20 μm, the interval Bof the adjacent hexagon is about 5 μm.

No crystal defects are observed on the surface of the second nitridesemiconductor 5 grown over the protective film except at the center ofthe hexagon.

EXAMPLE 8

The nitride semiconductor substrate is grown similarly to Example 6,except for the doping material for the first and second nitridesemiconductor layer 4 and 5. The first nitride semiconductor layer 4 isgrown without doping impurity material and the second nitridesemiconductor layer is grown with SiH4 to dope Si as impurity material.

Satisfactory result substantially similar to Example 6 is obtained.

EXAMPLE 9

The nitride semiconductor substrate is grown similarly to Example 8,except for the doping material for the second nitride semiconductorlayer 5. The second nitride semiconductor layer 5 is grown with CP₂Mgdoping Mg as impurity material.

Satisfactory result substantially similar to Example 8 is obtained.

EXAMPLE 10

The nitride semiconductor substrate is grown similarly to Example 8,except for the doping material for the second nitride semiconductorlayer 5 The second nitride semiconductor layer 5 is grown with SiH₄ andCP₂Mg to dope Si and Mg impurity material.

Satisfactory result substantially similar to Example 8 is obtained.

EXAMPLE 11

The nitride semiconductor substrate is grown similarly to Example 8,except for the doping material for the second nitride semiconductorlayer 5. The second nitride semiconductor layer 5 is grown withoutdoping impurity material.

Satisfactory result substantially similar to Example 8 is obtained.

COMPARATIVE EXAMPLE 1

A sapphire substrate 1 having principal plane in C plane and orientationslat surface in A plane is used. A buffer layer of GaN is formed to athickness of 200 Å on the sapphire substrate 1 at a temperature of 510°C. by MOCVD process using hydrogen as the carrier gas and ammonia andTMG (trimethyl-gallium) as the stock material gas.

After growing the buffer layer, supply of only the TMG is stopped andthe temperature is raised to 1050° C. When the temperature has reached1050° C., nitride semiconductor 2 made of undoped GaN is grown to athickness of 2.5 μm by using TMG, ammonia and silane gas as the stockmaterial gas.

A protective film 3 made of SiO₂ is formed to a thickness of 0.5 μm bythe CVD process on the nitride semiconductor 2 and, after forming aphoto-mask of stripe configuration, the protective film 3 made of SiO₂having stripe width of 14 μm and window size of 6 μm is formed byetching. Direction of the stripes of the protective film 3 is setperpendicular to the sapphire A plane.

Then the first nitride semiconductor 4 made of GaN is grown to athickness of 15 μm at a temperature of 1050° C. under a reduced pressureby MOCVD process using TMG, ammonia, silane gas and Cp₂Mg(cyclopentadienyl magnesium) as the stock material gas. In this process,the first nitride semiconductor 4 is grown from the window of the SiO₂protective film and laterally grown on the protective film. The growthis continued until the first nitride semiconductor 4 completely coversthe SiO₂ protective film.

CL (cathode luminescence) observation of the surface of the secondnitride semiconductor 4 is shown in FIG. 14B. Defects are observed notonly over the windows of the protective film, but also at the joint ofthe nitride semiconductor 4.

While the present invention has been fully described in conjunction withthe preferred embodiments by making reference to the accompanyingdrawings. It will be apparent for those skilled in the art that variousmodifications and alterations can be made. It should be understood thatsuch modifications and alterations which do not deviate from the spiritand scope of the present invention are included in the present inventionwhich is defined by the appended claims.

What is claimed is:
 1. A method for manufacturing a nitride semiconductor substrate comprising: forming a protective film including a plurality of windows of stripe, grid and/or island configuration on a supporting substrate; laterally growing a first nitride semiconductor layer over said protective film from a plurality of exposed portions of said supporting substrate, the growth being stopped in such a state that films of said first nitride semiconductor layer growing laterally from respective starting points do not join together but oppose each other via respective clearances; removing at least a part of said protective film; and growing a second nitride semiconductor layer laterally from the top surface or the top and side surface of said first nitride semiconductor layer to cover clearance(s) defined in said first nitride semiconductor layer so that cavities are formed below respective portions of the second nitride semiconductor layer which cover clearances defined in said first nitride semiconductor layer.
 2. The method for manufacturing a nitride semiconductor substrate according to claim 1, wherein said supporting substrate is formed by growing a nitride semiconductor over the entire surface of a substrate made of different material.
 3. The method for manufacturing a nitride semiconductor substrate according to claim 1, wherein said protective film is removed by etching or peeling off.
 4. The method for manufacturing a nitride semiconductor substrate according to claim 1, wherein said protective film is removed until said supporting substrate is exposed.
 5. The method for manufacturing a nitride semiconductor substrate according to claim 1, wherein said protective film is made of silicon oxide, silicon nitride, titanium oxide, or zirconium oxide or a multi-layered film of these materials, or a metal film which has a high melting point of 1200° C. or higher.
 6. The method for manufacturing a nitride semiconductor substrate according to claim 1, further comprising removing said supporting substrate partially or totally.
 7. The method for manufacturing a nitride semiconductor substrate according to claim 1, further comprising: forming a middle layer made of nitride semiconductor having the thickness of not less than 10 μm on said second nitride semiconductor layer; forming a second protective film having plurality of windows of stripe, grid and/or island configuration on said middle layer; and laterally growing a third nitride semiconductor over said second protective film from a plurality of exposed portions of said middle layer, the growth being stopped in such a state that films of said third nitride semiconductor layer growing laterally from respective starting points do not join together but oppose each other via respective clearances.
 8. The method for manufacturing a nitride semiconductor substrate according to claim 7, further comprising removing said supporting substrate partially or totally. 